Valve assemblies are used to regulate the transmission and distribution of fluid in various processes. Typical valve assemblies include a valve having a valve member that is movable to open and close the valve and an actuator having a movable output member (e.g., a stem) configured to drive movement of the valve member. Certain valve assemblies, most commonly pneumatically actuated valve assemblies, also include a valve positioner for controlling the valve actuator.
A valve positioner controls the actuator based on control signals received from a centralized process controller and/or feedback control that is processed locally by a processor of the positioner and typically includes a feedback shaft that is linked to the output member of the actuator for movement that corresponds with opening and closing of the valve. The valve positioner includes one or more position sensors that are configured to sense the position of the feedback shaft and output a corresponding position signal. A control processor (e.g., a centralized process controller or a control processor local to the valve positioner) receives the position signal and uses the position signal to determine whether the valve position should be changed. When the valve position must be changed, the control processor sends a control signal to the valve positioner and the valve positioner adjusts the position of the valve using the actuator.
Various types of actuators are known in the art. For example, some valve assemblies use pneumatic actuators, hydraulic actuators, electronic actuators, etc. For pneumatic actuators, a positioner is operatively connected to the actuator via a pneumatic connection. Based on the control signal, the valve positioner actuates the pneumatic actuator to adjust the position of the valve, thereby adjusting the position of the valve member to throttle the flow of process fluid through the valve to the desired amount.
In some instances, particularly when a valve is controlled by a centralized process controller, it may be desirable to provide redundant feedback information about the position of a valve. In such situations, a limit switch assembly or limit switch box can be operatively installed in the valve assembly. A limit switch assembly includes a feedback member that is operatively connected to the output member of the valve actuator for movement in response to movement of the valve actuator. Such a limit switch assembly can be operatively connected to the output member of the actuator using a linkage that is separate from the linkage that connects the feedback shaft of the valve positioner to the output member, or alternatively the feedback member can be directly connected to the feedback shaft of the valve positioner so that the limit switch assembly is appended to the valve positioner. A conventional limit switch assembly includes a position sensor for detecting the feedback member when it is moved to one or more predetermined limit positions. Typically, the limit switch assembly is calibrated so that a position sensor detects when the feedback member is positioned at a positon that corresponds with a position of the output member of the actuator when the valve is either fully open or fully closed. Calibrating a limit switch assembly involves adjusting the relative positions of the position sensor and the feedback member so that the position sensor detects the feedback member at the fully opened and/or fully closed position, which entails opening a limit switch assembly housing to access a calibration mechanism in the interior of the limit switch assembly.